A double magnetic tunnel junction (MTJ) stack for use in spin-transfer torque magnetoresistive random access memory (STT MRAM) is provided. The double MTJ stack includes a bottom tunnel barrier layer having hexagonal symmetry and composed of AlN, a magnetic free layer stack containing a synthetic anti-ferromagnetic coupling layer, and a top tunnel barrier layer having cubic symmetry. For such a double MTJ stack, the symmetry requirements for the tunnel barrier layers do not conflict anymore with the symmetry requirements for strong anti-ferromagnetic exchange. Thus, such a double MTJ stack can be used to provide performance enhancement such as faster switching times and lower write errors to a STT MRAM.